This invention relates to the reduction of sulfur oxide emissions from regenerators associated with catalytic cracking units and is particularly concerned with reducing the emissions of sulfur oxides utilizing a sorbent containing rare earth constituents.
Fluidized catalytic cracking (FCC) units are used in the petroleum industry to convert high boiling hydrocarbon feedstocks to more valuable hydrocarbon products, such as gasoline, having a lower average molecular weight and a lower average boiling point than the feedstocks from which they were derived. The conversion is normally accomplished by contacting the hydrocarbon feedstock with a moving bed of catalyst particles at temperatures ranging between about 800.degree. F. and about 1100.degree. F. The most typical hydrocarbon feedstock treated in FCC units comprises a heavy gas oil, but on occasion such feedstocks as light gas oils, naphthas, reduced crudes and even whole crudes are subjected to catalytic cracking to yield low boiling hydrocarbon products.
Catalytic cracking in FCC units is generally accomplished by a cyclic process involving separate zones for catalytic reaction, steam stripping, and catalyst regeneration. The hydrocarbon feedstock is blended with an appropriate amount of catalyst particles to form a mixture that is then passed to a catalytic reactor, normally referred to as a riser, wherein the mixture is subjected to a temperature between about 800.degree. F. and about 1100.degree. F. in order to convert the feedstock into gaseous, lower boiling hydrocarbons. After these lower boiling hydrocarbons are separated from the catalyst in a suitable separator, such as a cyclone separator, the catalyst, now deactivated by coke deposited upon its surfaces, is passed to a stripper. Here the deactivated catalyst is contacted with steam to remove entrained hydrocarbons that are then combined with the vapors exiting the cyclone separator to form a mixture that is subsequently passed downstream to other facilities for further treatment. The coke-containing catalyst particles recovered from the stripper are introduced into a regenerator where the catalyst is reactivated by combusting the coke in the presence of an oxygen-containing gas, such as air, at a temperature which normally ranges between about 1000.degree. F. and about 1500.degree. F. The cyclic process is then completed by blending the reactivated catalyst particles with the feedstock entering the riser or reaction zone of the FCC unit.
A major problem associated with FCC units occurs when the hydrocarbon feedstock contains organic sulfur compounds. The sulfur compounds in such a feedstock are converted to hydrogen sulfide in the catalytic reaction zone and stripping zone so that the bulk of the sulfur in the feedstock is recovered as hydrogen sulfide with the product vapors and later separated therefrom in downstream facilities, normally by contact with an aqueous alkanolamine solution. Some of the sulfur components, however, remain, or are converted to forms which remain, with the coke on the deactivated catalyst recovered from the stripper. Thus, when the coke is combusted in the regenerator, a flue gas containing sulfur oxide compounds is generated. This flue gas, if untreated, is a source of pollution. Although between 90 and 95 percent of the sulfur compounds entering an FCC unit with the feedstock are ultimately removed as hydrogen sulfide and other gaseous sulfur compounds in the reactor and stripper, the remaining 5 to 10 percent left with the coke and converted to sulfur oxide compounds in the regenerator represents a significant environmental and engineering problem.
In order to avoid environmental problems associated with the emissions of sulfur oxides from FCC units, various procedures have been suggested to reduce such emissions to environmentally tolerable levels. One such procedure involves circulating with the catalyst particles in the FCC unit a metal-containing component sometimes referred to as a "sulfur getter" that reacts in the regenerator with the gaseous sulfur oxide compounds to yield a spent "sulfur getter" containing solid sulfur compounds. The spent "sulfur getter" is then reconverted to an active sorbent by passage through the riser and stripper wherein the solid sulfur compounds are converted to hydrogen sulfide. The hydrogen sulfide is then recovered with the low-boiling hydrocarbons produced in the stripper and riser and passed to downstream units where the hydrogen sulfide is separated therefrom.
Normally, the "sulfur getter" or sulfur sorbent utilized in the catalytic cracking process must be prepared by processes involving substantial manufacturing costs. In an effort to reduce such costs, U.S. Pat. Nos. 4,311,581; 4,341,661; and 4,366,083, all of which are hereby incorporated by reference in their entireties, teach the use of bastnaesite, an abundant and inexpensive material, as a sulfur sorbent for removing sulfur oxides from the flue gases produced in FCC regenerators. It has been found, however, that although bastnaesite is initially a very active sorbent, its initial activity decays rapidly with repeated cycling from the riser and stripper to the regenerator and back again.
Accordingly, it is one of the objects of the present invention to provide a sulfur sorbent derived from bastnaesite that will retain its activity for sulfur oxides removal during cyclic catalytic cracking operations. It is another object of the invention to provide a sulfur sorbent of increased activity for reducing emissions of sulfur oxides during cyclic catalytic cracking operations. These and other objects of the invention will become more apparent in view of the following description of the invention.